The present invention relates generally to the field of post-manufacturing testing of microelectronic devices. More particularly, the present invention is directed to a system for and method of testing a microelectronic device using probe card having two probes for contacting each contact pad of the microelectronic device.
As manufacturers continually reduce the size of microelectronic devices contained in very large scale integration (VLSI) integrated circuits (ICs), it is becoming more difficult to test these devices to determine whether or not they function properly. This is so because as the size of the devices decreases, the electrical resistance through these devices also decreases. Therefore, the sensitivity of the test measurements, and, relatedly, accuracy of the electrical signals reaching the devices during testing, must increase accordingly.
However, the use of copper-based metallurgy in microelectronic devices increases the difficulty of providing the devices with an accurate signal. Unlike test connections made to the aluminum probe pads of microelectronic devices having aluminum-based metallurgy, test connections made to copper probe pads are problematic due to the formation of layers of copper oxide on the probe pads and the test probes. These copper oxide layers increase the contact resistance between the test probes and probe pads, decreasing the voltage applied across the devices. The reduction in voltage decreases the accuracy and sensitivity of the measurements made during testing and often leads to false failure determinations.
The resistance caused by the layers of copper oxide and other materials is commonly referred to as xe2x80x9ccontamination resistance.xe2x80x9d Various systems and methods have been developed for measuring contamination resistance. For example, Japanese Publication No. 11-133075 is directed to a system for and method of determining whether or not the contamination resistance of one or more probe pads is too large to obtain useful measurement data from a device wider test (DUT). The system comprises a probe card having a plurality of probes, or needles, for testing a DUT having a plurality of probe pads. Thc probe card provides a pair of probes for contacting each probe pad of the DUT. The pair of probes associated with each probe pad are spaced from one another and contact the cones on be ad at different locations.
The resistance caused by the layers of copper oxide and other materials is commonly referred to as contamination resistance. Various systems and methods have been developed for measuring contamination resistance. For example, Japanese Publication No. 11-133075 is directed to a system for and method of determining whether or not the contamination resistance of one or more probe pads is too large to obtain useful measurement data from a device under test (DUT). The system comprises a probe card having a plurality of probes, or needles, for testing a device under test (DUT) having a plurality of probe pads. The probe card provides a pair of probes for contacting each probe pad of the DUT. The pair of probes associated with each probe pad are spaced from one another and contact the corresponding probe pad at different locations.
The method disclosed in Japanese Publication No. 11-133075 includes passing a current via the pair of probes through the probe pad to determine the contact resistance, which is substantially equal to the contamination resistance. If the contact resistance is higher than a predetermined value, further testing of the DUT does not take place, since any measurement made in the presence of the excessive contact resistance would fall outside the acceptable range. A drawback of this method is that testing is a two-stage process. First, the Kelvin testing is performed on each probe pad to determine the level of contamination resistance. Then, if the results of the Kelvin probing are satisfactory, testing of the devices in the DUT proceeds. Another drawback of the method is that the determination made is only binary. Further testing is either performed or not based upon the magnitude of the contact resistance.
A similar method of checking contact resistance during testing of a DUT is disclosed in U.S. Pat. No. 5,999,002 to Fasnacht et al. Fasnacht et al. disclose that contact resistance may be measured using a Kelvin connection and an impulse pulse generated by a transformer driven by a microprocessor. Although Fasnacht et al. state that contact resistance testing may be made concurrently with testing of the IC contained within the DUT, similar to the method disclosed in Japanese Publication No. 11-133075, the results are still binary. Either the contact resistance is too high and testing of the IC produces false results or contact resistance is within an acceptable range and testing of the IC produces acceptable results.
In one aspect, the present invention is directed to a system for testing a DUT having a plurality of probe pads. The system comprises a forcing probe for contacting and applying a first electrical signal to a first one of the plurality of probe pads. A sensing probe is provided for contacting the first one of the plurality of probe pads and sensing a second electrical signal at the first one of the plurality of probe pads. A variable power supply is in electrical communication with the forcing probe and the sensing probe. The variable power supply is capable of adjusting the first electrical signal based upon the second electrical signal.
In another aspect, the present invention is directed to a method of testing a DUT having a plurality of probe pads. The method includes the steps of providing a first electrical signal to one of the plurality of probe pads. A second electrical signal is then sensed at the one of the plurality of probe pads. The first electrical signal is adjusted based upon the second electrical signal.